This invention is an improved process for the production of acetonitrile. More particularly, this invention is an improved process for the purification of crude acetonitrile produced, for example, as a byproduct during the formation of acrylonitrile by the ammoxidation of an olefin such as propylene or the ammoxidation of propane. This invention is also a process for the purification of recycled and waste solvent acetonitrile.
Acetonitrile is a solvent used in a variety of industrial chemical processes and is a solvent of choice for use in high performance liquid chromatography, usually in combination with one or more solvents such as water or an alcohol. Acetonitrile is also used as a raw material for the preparation of other chemicals used in the chemical and pharmaceutical industry. Pure acetonitrile is desirable for such processes and other uses.
An important source of acetonitrile is a byproduct stream produced during the manufacture of acrylonitrile by the catalytic ammoxidation of propylene such as the process disclosed in U.S. Pat. No. 5,093,299. The catalytic ammoxidation of propylene is a major industrial process and the byproduct stream containing acetonitrile is produced worldwide in hundreds of millions of pounds per year. Although the amounts may vary, a typical example of this byproduct stream contains approximately 50% acetonitrile, 40% water, and smaller amounts of hydrogen cyanide (HCN), acrylonitrile, and other organic materials such as oxazole, allyl alcohol, acetone, and propionitrile. Prior processes for separating the desired acetonitrile from the other components in the byproduct mixture, particularly from water, are not simple. For example, in the process disclosed in U.S. Pat. No. 4,362,603, three different distillation procedures were employed. The first is a high pressure fractional distillation to remove lights and heavies, the second is a low pressure distillation to form an overhead of a water-acetonitrile azeotrope and a heavy fraction which includes water, and the third is a high pressure distillation designed to produce a middle cut or side-draw of relatively pure acetonitrile, a water-acetonitrile overhead and a bottoms or heavy fraction. While this process produces relatively pure acetonitrile, it requires the use of three distillation steps and the attendant recycle streams. The art needs a simpler process for the purification of crude acetonitrile, particularly acetonitrile that is mixed with water or other solvents. The present invention provides such an improved process.
This invention is a method for the purification of crude or impure acetonitrile comprising distilling the acetonitrile in a fractional distillation column at below atmospheric pressure, withdrawing a first side draw fraction comprising acetonitrile, distilling the first side draw fraction in a second fractional distillation column at super atmospheric pressure, and withdrawing from the second distillation a second side draw fraction comprising purified acetonitrile.
The acetonitrile used in the method of this invention is preferably acetonitrile produced as a byproduct during the ammoxidation of propylene or other suitable hydrocarbon such as propane. However, the method of this invention can be used to purify any source of acetonitrile including, for example, recycle or waste acetonitrile. Such recycle or waste acetonitrile may be acetonitrile that has been used as a solvent in other manufacturing processes, or as a solvent for high pressure liquid chromatography and may contain as impurities various amounts of water and one or more organic impurities.